1. Field of the Invention
The present invention is related to a focus detection device or distance detection device equipped with an image sensor, wherein the electric charge accumulation time of the image sensor is controlled by a characteristic value of a subject image signal.
2. Description of the Related Art
Focus detection devices and distance detection devices using image sensors are known and used in cameras. These devices typically perform the following steps: (1) subject image signals, relating to accumulated electrical charges in a charge coupled type image sensor, are output in series from the image sensor to a microcomputer, that is, the electric charges are "read" by the microcomputer; (2) the subject image signals undergo analog-to digital (AD) conversion by an AD converter built into the microcomputer, the subject image signals thereby becoming subject image data; and (3) the microcomputer then computes and processes the subject image data and calculates the focus condition and the subject distance.
For the next reading of electrical charges (that is, when step 1 is repeated), the electric charge accumulation time of the image sensor is controlled in relation to characteristic values of the subject image data. Such characteristic values can include peak value, average value and/or contrast value. By controlling the accumulation time in relation to a characteristic value, the level of the subject image signals are optimized for the focus condition and distance detection.
In a conventional system, the microcomputer determines the characteristic value of the image signals. This function of determining the characteristic value is not included as part of the image sensor.
When controlling the accumulation time of the charge coupled type image sensor for the next reading based on the current reading of subject image data, it is necessary to continuously operate the image sensor while (a) incorporating the subject image data into the microprocessor via analog-to-digital conversion; and (b) controlling the discharge time of the image sensor so that the subject image signal levels for the focus condition and distance detection are accurate.
Even if focus or distance is not being detected, the image sensor must be continuously operated and steps (a) and (b) must be continuously performed until the subject image signal levels become accurate. This continuous operation is necessary to provide adequate responsiveness when focus detection or distance detection is activated. If the image sensor is not continuously operated and steps (a) and (b) are not continuously performed, the responsiveness of focus detection and distance detection can be inadequate. Therefore, all subject image signals from the image sensor must constantly undergo analog-to-digital conversion and the subject image data must be constantly incorporated into the microcomputer in order to prepare for initiating focus detection and distance detection. As a result, even if focus detection and distance detection are not being performed, the microcomputer must continue to perform analog-to-digital conversion.
Therefore, processing power which could be useful to perform other functions (for example, exposure control) is not available and, instead, must be used to perform analog-to-digital conversion. Moreover, some devices may be capable of focus and distance detection in multiple regions. These devices can typically select one focus detection region from among multiple focus detection regions. However, even if focus and distance are being detected for only one selected focus detection region, a burden is placed on the microcomputer since the action of incorporating the subject image data for the focus detection regions which were not selected must still be performed. This incorporation of the subject image data for non-selected focus detection regions is performed in parallel with the focus detection of the selected focus detection region. As a result, the responsiveness of the focus detection and distance detection operations is decreased.